Metal-free flow battery to revolutionise renewable grids

A metal-free flow battery developed by American scientists promises to revolutionise the way electricity is stored on a grid.

The invention, described in the recent issue of Nature is hailed as an important breakthrough, paving the way for more economical and reliable storage of renewable energy including wind and solar.

The team from the Harvard School of Engineering and Applied Sciences (SEAS) has taken advantage of electrochemistry of naturally abundant, inexpensive, small organic (carbon-based) molecules called quinones, which are similar to molecules that store energy in plants and animals.

“The whole world of electricity storage has been using metal ions in various charge states but there is a limited number that you can put into solution and use to store energy, and none of them can economically store massive amounts of renewable energy,” said Roy G. Gordon, one of the paper’s authors. “With organic molecules, we introduce a vast new set of possibilities. Some of them will be terrible and some will be really good. With these quinones we have the first ones that look really good.”

More than 10,000 quinone molecules were screened to identify the best suitable candidates to be used in the innovative battery. Quinones are abundant in crude oil as well as in green plants. The molecule that the Harvard team used in its first quinone-based flow battery is almost identical to one found in rhubarb. The quinones are dissolved in water, which prevents them from catching fire.

Flow batteries store energy in chemical fluids contained in external tanks, instead of within the battery container itself. These batteries comprise of two main components - the electrochemical conversion hardware through which the fluids flow and the chemical storage tanks. The amount of energy that can be stored is limited only by the size of these tanks. The design permits larger amounts of energy to be stored at lower cost than with traditional batteries.

By contrast, in solid-electrode batteries, such as those commonly found in cars and mobile devices, the power conversion hardware and energy capacity are packaged together in one unit and cannot be separated. Consequently, they can maintain peak discharge power for less than an hour before being drained, and are therefore ill suited to store intermittent renewables.

“The intermittent renewables storage problem is the biggest barrier to getting most of our power from the sun and the wind,” said researcher Michael J. Aziz. “A safe and economical flow battery could play a huge role in our transition off fossil fuels to renewable electricity. I'm excited that we have a good shot at it.”

Recently, a growing number of engineers has been researching the flow battery technology. But until now, flow batteries have relied on chemicals that are expensive or difficult to maintain, driving up the energy storage costs.

The active components of electrolytes in most flow batteries have been metals. Vanadium is used in the most commercially advanced flow battery technology now in development, but its cost sets a rather high floor on the cost per kilowatt-hour at any scale. Other flow batteries contain precious metal electrocatalysts such as the platinum used in fuel cells.The new flow battery developed by the Harvard team already performs as well as vanadium flow batteries, with chemicals that are significantly less expensive, and with no precious metal electrocatalyst.

In practice, the technology could be deployed in a form of huge storage tanks located below ground level next to commercial wind turbines or below a roof covered with solar panels.

The team is now working with a Connecticut-based company Sustainable Innovations, LLC, to build a demonstration facility containing a unit the size of a horse trailer.

“You could theoretically put this on any node on the grid,” Aziz said. “If the market price fluctuates enough, you could put a storage device there and buy electricity to store it when the price is low and then sell it back when the price is high. In addition, you might be able to avoid the permitting and gas supply problems of having to build a gas-fired power plant just to meet the occasional needs of a growing peak demand,” he explained.